Regulation of the exhaust gas flow through the turbine of an exhaust gas-driven turbocharger provides known operational advantages in terms of improved ability to control the amount of boost delivered by the turbocharger to the associated internal combustion engine. The regulation of exhaust gas flow is accomplished by incorporating variable geometry into the nozzle that leads into the turbine wheel. By varying the size of the nozzle flow area, the flow into the turbine wheel can be regulated, thereby regulating the overall boost provided by the turbocharger's compressor.
Variable-geometry nozzles for turbochargers generally fall into two main categories: variable-vane nozzles, and sliding-piston nozzles. Vanes are often included in the turbine nozzle for directing the exhaust gas into the turbine in an advantageous direction. Typically a row of circumferentially spaced vanes extend axially across the nozzle. Exhaust gas from a chamber surrounding the turbine wheel flows generally radially inwardly through passages between the vanes, and the vanes turn the flow to direct the flow in a desired direction into the turbine wheel. In a variable-vane nozzle, the vanes are rotatable about their axes to vary the angle at which the vanes are set, thereby varying the flow area of the passages between the vanes.
In the sliding-piston type of nozzle, the nozzle may also include vanes, but the vanes are fixed in position. Variation of the nozzle flow area is accomplished by an axially sliding piston that slides in a bore in the turbine housing. The piston is tubular and is located just radially inwardly of the nozzle. Axial movement of the piston is effective to vary the axial extent of the nozzle opening leading into the turbine wheel.
One type of a sliding-piston includes a divider that divides the nozzle into separate first-stage and second-stage nozzles. When the piston is in the closed position, the piston is adjacent to the radially inner (i.e., trailing) edge of the divider, thereby effectively closing the second-stage nozzle and causing the exhaust gas to flow to the turbine wheel area via the first stage-nozzle only. When the piston is in an open position, the gas can flow through both the first-stage and second-stage nozzles.
The sliding-piston type of variable nozzle offers the advantage of being mechanically simpler than the variable-vane nozzle. Nevertheless, other drawbacks have generally been associated with sliding-piston type variable nozzles. For example, the divider must be machined to precise dimensional tolerances to ensure proper sealing between the piston and the divider along a contact surface. The divider is relatively large, and thus a significant amount of precision machining must be performed. Further, even with precision machining, the contact surface between the piston and the divider still typically permits leakage of exhaust gas between the two members.
Additionally, when the piston is in the open position, the shape of the upstream and downstream portions of the divider do not direct the flow of exhaust gas to the turbine wheel in an advantageous direction. As a result of these drawbacks, turbine performance is reduced.